Method for methane oxidation and, apparatus for use therewith

ABSTRACT

The invention is a method and apparatus for use therewith for the combustion of methane. The method employs reformation of methane and oxygen in fuel-rich proportions into carbon monoxide and hydrogen and residual methane. The carbon monoxide, hydrogen and residual methane is then combined with oxidant in fuel lean proportions to continue oxidation in a porous media that absorbs some of the heat of oxidation and radiates the heat as infrared radiation.

FIELD OF THE INVENTION

[0001] The present invention is generally directed to catalyticcombustion, and more specifically to a method and an apparatus for usetherewith for the reformation of methane into partial oxidation productsand the oxidation of those products at a temperature below the adiabatictemperature thereof.

BACKGROUND OF THE INVENTION

[0002] Methane is an abundant hydrocarbon that is used as a source offuel in numerous applications, such as industrial radiant heaters, gasturbines, home furnaces and cooking equipment. While methane can be madeavailable in a relatively pure form, it is more commonly provided as aconstituent of natural gas, of which it is the primary component.

[0003] Natural gas is typically combusted in an open flame, a processreferred to as diffusion burning, which generates certain pollutants.One particularly undesirable class of pollutants formed during diffusionburning is nitrous oxides, i.e. NOx. In diffusion burning, NOx can beformed by any one of three possible mechanisms: thermal, prompt, andfuel bound. The production of NOx by the thermal and the promptmechanisms, however, far exceeds that produced from the fuel boundmechanism. Consequently, efforts to reduce NOx pollution focus onreducing NOx formation by the thermal and/or the prompt mechanisms.

[0004] NOx produced by the thermal mechanism, i.e. thermal NOx, is oftenthe dominant mechanism. Thermal NOx is formed when the heat beingreleased by diffusion burning is sufficient to provide the necessaryenergy for the nitrogen in the air to combine with the oxygen in theair. Generally, at flame temperatures below 1700 K, the production ofthermal NOx is insignificant. However, as flame temperatures increase,the production of thermal NOx increases sharply.

[0005] Thermal NOx production can be controlled by regulating reactantstoichiometry. To burn a fuel it must be mixed with an oxidant. Forexample, to burn methane oxygen must be provided. The ratio of the fueland oxidant, that is methane and oxygen, is the reactant'sstoichiometry. Reactant stoichiometry is expressed in terms of afuel/oxidant equivalence ratio, or where the oxidant is oxygen as aconstituent of air—fuel/air ratio. The fuel/oxidant equivalence ratio isthe ratio of the actual fuel/oxidant ratio to the stoichiometricfuel/oxidant ratio. For example in the case of methane (CH₄), thecombustion reaction is CH₄+2O₂→CO₂+2H₂O. Therefore, a stoichiometricfuel/oxidant ratio is one part CH₄ and two parts O₂. Thus, if a mixturehad this ratio of CH₄ and O₂, the reactant stoichiometry as expressed bythe fuel/oxidant ratio of the mixture would be 1.0 (an actual mixturehaving these proportions would be referred to as stoichiometric).

[0006] A mixture having an equivalence ratio greater than 1.0 is fuelrich, i.e., in the case of the above methane reaction more than one partfuel for each two parts of oxygen, and a mixture having an equivalenceratio less than 1.0 is fuel lean, i.e. in the case of the above methanereaction less than one part fuel for each two parts of oxygen. Whencombustion is adiabatic, stoichiometric mixtures burn relatively hotterthan non-stoichiometric mixtures and the further away the mixture isfrom stoichiometric the relatively cooler it burns.

[0007] NOx production by the prompt mechanism, i.e. prompt NOx, is afuel-rich, gas-phase phenomenon. The reaction is quick and completeswithin the diffusion flame. The production of NOx by the promptmechanism can only be controlled if the diffusion flame is eliminated,in whole or in part.

[0008] NOx formation from the combustion of methane could be greatlyreduced if methane could be combusted at temperatures below 1700 degreesK and diffusion flame could be avoided. It is well known in the art thatif methane is catalytically combusted, i.e. oxidized in the presence ofa catalyst, the energy within the methane can be released without theformation, or limited formation, of thermal and/or prompt NOx.

[0009] A problem, however, with the catalytic combustion of methane isthat methane is a very stable molecule. Thus, it is more difficult tooxidize than higher order hydrocarbons, such as propane. Methane can becatalytically combusted under fuel lean conditions producing combustiontemperatures below 1700 degrees K. When a palladium-based catalyst isused the reaction may become unstable due to properties of Pd-PdOtransformation of the catalyst. Hysteresis in the catalyst activitymakes controlling the reaction extremely difficult. Platinum basedcatalysts on the other hand can provide more stable operation. However,volatility of Pt at the desired temperatures under lean conditions isvery high. Thus, platinum catalyst lacks durability.

[0010] Based on the foregoing, it is an object of the present inventionto develop a method and an apparatus for the combustion of methane thatovercomes the problems and drawbacks associated with the prior art.

SUMMARY OF THE INVENTION

[0011] The present invention is directed in one aspect to a method forthe combustion of methane. In the method, a fluid stream including fuelhaving methane and oxygen that is in fuel rich proportions, i.e. havinga fuel/oxidant equivalence ratio greater than 1.0, is provided. Thefluid stream flows into a reformation reactor having a catalyst thereinthat promotes the reformation of methane (CH₄) into carbon monoxide (CO)and hydrogen (H₂).

[0012] The catalyst reforms at least a portion of the methane in thefluid stream into carbon monoxide and hydrogen creating an exhauststream exiting the reformation reactor having various fuel constituentstherein, such as unreformed methane, CO and H₂. The exhaust stream isthen divided into a plurality of exhaust streamlets by passing theexhaust stream into a manifold having a plurality of discretedischarges. As a portion of the exhaust gas exits through a discharge,an exhaust streamlet is formed. Sufficient oxygen is then added to theexhaust streamlet such that the fuel constituents therein and the oxygenare in fuel-lean proportions. Same amount of oxygen should be added toeach streamlet, such that variations in the equivalence ratios betweenthe streamlets are small. The exhaust and second fluid are addedtogether, not mixed. In the present invention, it is desired that theexhaust and the second fluid enter the porous media as distinct flowsteams. It is understood however, that the two fluids will be in contactalong an interface and that incidental diffusion of one fluid intoanother will occur. It is expected that if sufficient time is providedthe diffusion combustion would occur at the interface before the twostreamlets can mix. To avoid gas phase flame oxidation of the exhauststream, which is undesirable in this invention, combined stream formedafter adding the second stream to the exhaust stream should be passedinto the porous media before combustion takes place. Finally, at least aportion of the CO, H₂ and CH₄ in the exhaust streamlets is oxidized bypassing the combined stream through a porous media that absorbs and thenradiates some of the heat generated by the oxidation.

[0013] A catalytic burner suitable for performing the above methodincludes a reformation reactor incorporating a catalyst. A manifold thatreceives the exhaust stream from the reformation reactor and passes theexhaust stream through a plurality of discharges forming part of themanifold thereby creating a plurality of exhaust streamlets. The exhauststreamlets then enter a flow path where the exhaust streamlets aredirected into a proximally located porous media. Means for introducing asecond fluid into the flow path are also provided.

[0014] The reformation reactor is a partial oxidation reactor. In apartial oxidation reactor, the catalyst and its associated geometry,e.g. substrate and dispersion thereon, defines an activity relative tothe flow rate, i.e., residence time, of the methane/oxygen thereoversuch that when the catalyst and the methane/oxygen interact partialoxidation products and not complete oxidation products are predominantlyformed. In the case of methane and oxygen, partial oxidation productsare H₂ and CO, while the compete oxidation products are H₂O and CO₂. Anexample of a reformation reactor for methane suitable for thisapplication is disclosed in U.S. Pat. No. 5,648,582, the disclosure ofwhich is incorporated herein in its entirety.

[0015] As those skilled in the art will appreciate, the selectivity,i.e. the ability to produce one product in favor of another, in thereformation process can be manipulated by controlling the temperature ofthe fluid stream. In the case of a fluid stream including methane andoxygen in fuel rich proportions, preheating of the fluid streamincreases the selectivity in the reformation of methane in favor of H₂and CO versus CO₂ and H₂O. Therefore, an enhancement to both the methodand the catalytic burner incorporates heating the fuel stream prior toits entry into the reformation reactor.

[0016] The exhaust and the second fluid are mixing and reacting insidethe porous media to further oxidize at least part of the exhaust streamto the complete oxidation products. The porous media absorbs some of theheat created by the exothermic oxidation reaction and emits it in theform of IR radiation, assuring that the temperature remains below theadiabatic flame temperature defined by the reactant stoichiometry of thefuel constituents and oxygen. A porous media can be any media throughwhich a gas can flow, while continuously encountering solid surfaces. Inother words, porous media is comprised of alternating regularly orrandomly empty volumes and filled volumes. Empty volumes should form acontinuous network, such that the porous media remains permeable topermit the flow of a fluid therethrough. The porous media should have apore size, which describes the average size of the empty volume (if thepores size in not round the smaller dimension), that is generallyuniform, but small deviations are acceptable. Porous media having a fewlarge empty volumes and otherwise generally uniform smaller volumescould be problematic. The precise pore size, porosity (ratio of openvolume to total volume) and material is application dependent.

[0017] The material for the porous media should be chosen to withstandthe temperatures generated in the exothermic oxidation process andeffectively emit heat in the form of infrared radiation (IR). Pore sizeand porosity are chosen large enough to minimize pressure drop inducedby the porous media but small enough when compared to the total volumein which the oxidation reaction between the exhaust and the secondstream takes place.

[0018] As those skilled in combustion will readily appreciate, thereformation reactor requires that the catalyst therein be at a certaintemperature to perform the reformation. The catalyst can be brought tothis temperature by any one or a combination of well know procedures,such as heating the fluid stream, or direct heating of the catalyst.

[0019] Regardless of the method chosen, the exhaust gas will have atemperature upon exiting the catalyst equal to the operationaltemperature chosen for the reformation reactor plus the exothermalresulting from the exothermic oxidation process taking place therein. Itshould be remembered that the proportions of fuel constituents to oxygenwithin the exhaust stream are still be quite rich, i.e., the initialstream had fuel rich proportions and oxidant was consumed along withfuel creating a progressively richer fuel stream as it passed throughthe reformation reactor. Therefore, although the fuel constituents inthe exhaust gas will be quite hot, oxidation will not occur within theexhaust stream until additional oxidant is added.

[0020] Where the fuel/oxygen stoichiometry, flow rate and IR radiationare such that porous media is hot enough, oxidation of fuel inside theporous media will occur upon contact with an oxidant. Where the porousmedia is not hot enough to support oxidation on contact with an oxidant,the porous media can utilize a suitable oxidation catalyst to sustainthe oxidation reaction. It is understood that a catalyst can be usedeven if the fuel constituents are hot enough to support combustion.

DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a side view of the catalytic burner of the presentinvention.

[0022]FIG. 2 is a top view of the manifold of the present inventiontaken along line 2-2 of FIG. 1.

[0023]FIG. 3. is a side view of a second embodiment of the catalyticburner of the present invention.

[0024]FIG. 4 is a top view of the catalytic burner taken along the line4-4 of FIG. 3 showing the heat exchanger.

DETAILED DESCRIPTION

[0025] As shown in FIG. 1, the catalytic burner, generally referred toby the reference number 10, is comprised of a reformation reactor 12, amanifold 14 and a porous media 16. An inlet stream 18 enters thereformation reactor 12 by means of a flow path 20 creating an exhauststream 24. The manifold 14 and reformation reactor 12 are connected by aflow path 26 such that the exhaust stream 24 enters the manifold 14 toexit through a plurality of discharges 28 (See FIG. 2). Exhauststreamlets 30 are formed by the discharges 28. The discharges 28 arepositioned proximate the porous media 16, and connected by a flow path32, such that upon exiting the discharges 28 the exhaust streamlets 30enter an inlet face 15 of the porous media 16.

[0026] An oxidant 34 flows around the manifold 14 permitting the oxidant34 to flow into the flow path 35 connecting the discharges 24 to theporous media 16. As shown in FIG. 2, the discharges 28 are positioned todisperse uniformly the exhaust stream 24 as exhaust streamlets 30 over adispersion area 36, defined by a perimeter 38. The hub and spoke designof the manifold 14 assists in distributing the exhaust streamlets 30uniformly under the porous media 16 across the inlet face 15, but themanifold 14 and discharges 28 therefrom could be of any design, such asport injectors positioned in the housing 12, thus the invention shouldnot be considered limited to the manifold 14 shown.

[0027] The flow path 32 and manifold 14 should cooperate to uniformlydisperse the oxidant 34 and exhaust stream 24 across the inlet face 15of the porous media 6. It is a feature of this invention that theoxidant 34 and exhaust streamlets 30 be associated, but not mixed in theflow path 32. Associated means that the exhaust streamlets 30 andoxidant 34 are brought in contact but are not provided sufficient timeto inter-defuse, and therefore, the exhaust streamlets 30 and oxidant 34generally enter the porous media 16 as discrete streams.

[0028] In the method of the present invention, the inlet stream 18 hasmethane and oxygen in fuel rich proportions. Preferably, the oxygen isprovided as a constituent of air. If desired, the methane can beprovided as a constituent of a blended fuel, such as natural gas.Preferably, the methane and oxygen are highly mixed. The operationalperimeters of the reformation reactor 12, including the catalysttherein, are selected such that some, or for all practical purposes all,of the methane is converted primarily into CO and H₂ instead of CO₂ andH₂O. This creates an exhaust stream 24 from the reformation reactor 12having therein at least the fuel constituents CO, and H₂.

[0029] The fuel constituents in the exhaust stream 24 define anadiabatic temperature. The exhaust stream 24 is then divided intoexhaust streamlets 30. The exhaust streamlets 30 are then associatedwith additional oxygen, generally as a constituent of air, in fuel leanproportion (exhaust stream to oxygen). It is preferred that exhaust andoxygen are mixed in a proportion close to stoichiometric with smallexcess oxygen. The exhaust streamlets 30 and additional oxygen then passinto the porous media 16 where mixing and oxidation, which isexothermic, takes place. The porous media 16 is constructed of materialsthat absorb some of the heat of reaction, such that the oxidationoccurring in the porous media 16 is below the adiabatic temperature ofthe fuel constituents. The heat of reaction absorbed by the porous media16 is radiated therefrom in the form of infrared radiation.

[0030] As discussed above and shown in FIGS. 1 and 2, the catalyticburner 10 has a plurality of discharges 28 that divide the exhauststream 24 into exhaust streamlets 30. In the context of the method, theexhaust stream 24, which is in fuel rich proportion, has associated withit a certain amount of energy. The energy density of the exhaust streamis proportional to the amount of fuel passing through a certaincross-sectional area per unit of time, i.e. to the volumetric flow rateof the exhaust stream 24. U.S. Pat. No. 5,648,582 suggests that oneessential feature of the reformation reactor 12 is that the inlet stream18 enters the reactor at very high space velocity and the reformationreaction occurs at short residence time. This provides that flow spacevelocity and associated energy density in the exhaust stream 24 willalso be high. If the exhaust stream 24 were to be exposed to additionaloxidant as a single stream, excessive amount of heat, associated withthe oxidation reaction, would be released in a small volume of theporous media 16. This excessive heat could cause deterioration, orfailure, of the porous media 16. The manifold 14 distributes the exhauststream 24 over the larger cross-sectional area, effectively decreasingthe energy density in the stream. The energy density associated withindividual exhaust streamlets 30 and any diffusion flame that maybeassociated therewith is considerably lower and may be adjusted dependingon the application. The discharges 28 can also act as diffusers toreduce further the power density, i.e., power per area, of the exhauststream 24.

[0031]FIG. 3 is a second embodiment of the catalytic burner which issimilar to the previous embodiment, therefore, like reference numberpreceded by the number 1 are used to indicate like elements. In thisembodiment, the catalytic burner 110 is positioned in an interior area140 of a housing 142. Also positioned within the interior area 140 is aheat exchanger 144. The reformation reactor 112 is positioned within theporous media 116 as opposed to under it. In this embodiment, the inletstream 118 enters a heat exchanger 144 positioned within the interiorarea 140 adjacent the porous media 116. The porous media 116 has acatalyst 146 deposited on the surface thereof. The catalyst 146 isselected to support the continued oxidation of the H₂, CO and CH₄ in theexhaust streamlets 130. The inlet stream 118 flows through the heatexchanger 144 prior to entering the reformation reactor 112.

[0032] As explained above, in the method of the present invention anoxidation reaction occurs in the porous media 116. As such, some of theheat of reaction 147 leaves the porous media 116 and is conducted intocontact with heat exchanger 144, where some of the heat of reaction istransferred into the inlet stream 118 flowing therein. Referring to FIG.4, the heat exchanger 144 is comprised of a tube 148 that has beenformed into a flat coil about a center point on an axis designated bythe letter A.

[0033] The heat exchanger 144 could be of any other design, which allowspart of heat released in porous media 116 to be transferred into theinlet stream 118, thus, the invention should not be considered limitedto the heat exchanger 144 shown.

[0034] Continuing with FIG. 3, the manifold 114 is adapted to receivethe exhaust stream 130 from the reformation reactor 112. In thisembodiment, the means for introducing additional oxidant 134 between thedischarges 128 and the porous media 116 is by the introduction ofadditional oxidant 134 into the housing 142 below the discharges 128.Depending upon the method of operation, the flow of additional oxidant134 may be by natural convection or a pump, such as a fan. In mostcases, the introduction point is not critical as oxygen as a constituentof air will be the oxidant 134 and the air will naturally flow to thedesired location. Therefore, the means could include passages in thehousing, or the additional oxidant 134 could flow from a point above theporous media 116 into the housing 142.

[0035] In this embodiment, the reformation reactor 112 is shownpositioned within the porous media 116. This is not a requirement of theinvention, as the reformation reactor 112 could be positioned anywhereincluding outside the interior area 40.

[0036] In the method of the present invention, this embodiment isdesigned to provide the additional step of preheating of the inlet gasstream 118 using some of the heat of reaction produced by the exothermicreaction in the porous media 116. Preheating the inlet stream 118 offersthe advantage of increasing the selectively to CO and H₂ within thereformation reactor 12. This is but one method of preheating, thereforethe invention should not be considered so limited. Preheating of theinlet stream 118 by other means such as electric resistance areconsidered within the scope of the invention. Preheating of the inletstream can assist in starting the catalytic burner.

[0037] The porous media 16, 116 is a media through which a gas can flow.In the preferred embodiment, the porous media 16, 116 was made from aplurality of stacked short-channel screens. The invention should not beconsidered so limited however, as other media could be used such aspellets, foams or gauzes and even a single screen. Generally, porousmedia are graded by “pore size.” Another important parameter for thisinvention, however, is consistency of pore size. The porous media 16,116 is designed to promote interaction of the fuel constituents withinthe exhaust stream 24 with the additional oxidant 34, 134, extract heatfrom the ongoing oxidation, and radiate infrared radiation. Further, theporous media 16, 116 continually assures that the exhaust stream 24, 124and oxidant 34, 134 are divided into small pockets. In other words, theexhaust stream 24, 124 and oxidant 34, 134 cannot reform into a largevolume. These requirements mean that preferably the pores within theporous media 16, 116 are generally uniform. Pore size is chosen suchthat the pores are large enough to minimize pressure drop but smallenough to assure an acceptable heat release within a pore.

[0038] Although the present invention has been described in considerabledetail with reference to certain preferred versions thereof, otherversions are possible, particularly versions having more than twocatalysts. Therefore, the spirit and scope of the invention should notbe limited to the description of the preferred versions containedherein.

What is claimed is:
 1. A method for combustion of a fuel includingmethane, the method comprising the steps of: providing a fluid streamthat includes fuel and oxygen in fuel rich proportions to a reformationreactor having a catalyst therein upon which at least a portion of thefuel stream contacts; reforming, via catalytic reaction, at least aportion of the methane in the fuel into carbon monoxide and hydrogen toform an exhaust stream having various fuel constituents therein;associating oxygen with the exhaust stream, the oxygen being provided inquantities that cause the ratio of oxygen and the fuel constituents inthe exhaust stream to be in fuel lean proportions; oxidizing at least aportion of the fuel constituents in the exhaust stream within a porousmedia creating a heat of reaction; and radiating at least a portion ofthe heat of reaction from the porous media.
 2. The method of claim 1having the additional step of preheating the fluid stream prior to thereforming step.
 3. The method of claim 1 wherein the porous media has acatalyst positioned on the surface thereof.
 4. A method for combustionof a fuel including methane, the method comprising the steps of:providing a fluid stream including fuel and oxygen in fuel richproportions; reforming at least a portion of the methane in the fluidstream into carbon monoxide and hydrogen to create an exhaust streamfrom the catalyst having various fuel constituents therein; associatingoxygen with the exhaust stream, the oxygen having a volume in fuel leanproportions to the fuel constituents within the exhaust stream;oxidizing at least a portion of the fuel constituents within a porousmedia creating a heat of reaction; and radiating at least a portion ofthe heat of reaction from the porous media.
 5. The method of claim 4having the additional step of preheating the fluid stream prior to thereforming step.
 6. The method of claim 4 having an additional step ofdispersing the exhaust stream prior to associating with oxygen.
 7. Acatalytic burner comprising: a reformation reactor having a catalysttherein suitable for the converting of at least a portion of the methanein the fuel stream including methane and oxygen in fuel rich proportionsto carbon monoxide and hydrogen for creating an exhaust stream; amanifold having a plurality of discharges, the manifold in fluidcommunication with the exhaust stream and defining a pluralitydischarges; a porous media: means defining a flow path between at leastsome of the discharges and the porous media; and means for introducingoxygen into the flow path such that the exhaust stream and oxygen are infuel lean proportions.
 8. The catalytic burner of claim 7 wherein theporous media is a plurality of stacked short-channel screens.
 9. Thecatalytic burner of claim 7 wherein the porous media has a catalystpositioned on the surface thereof.
 10. The catalytic burner of claim 7further comprising a heat exchanger, downstream of the porous media forreceiving and passing the fuel therethrough.
 11. The catalytic burner ofclaim 8 wherein the heat exchanger is a spiral shape tube.
 12. Thecatalytic burner of claim 7 wherein the manifold includes a hub having aplurality of spokes extending therefrom.
 13. The catalytic burner ofclaim 7 wherein at least a portion of the reformation reactor ispositioned within the porous media.
 14. A catalytic burner comprising: areformation reactor for reforming an inlet stream; a manifold having aplurality of discharges; means defining a first flow path between thereformation reactor and the manifold; a porous media: means defining asecond flow path between at least some of the discharges and the porousmedia; and means for introducing oxidant into the second flow path. 15.The catalytic burner of claim 14 wherein the porous media is a pluralityof stacked short-channel screens.
 16. The catalytic burner of claim 14wherein the porous media has a catalyst positioned on the surfacethereof.
 17. The catalytic burner of claim 14 further comprising a heatexchanger located downstream of the porous media.
 18. The catalyticburner of claim 17 wherein the heat exchanger is a spiral shape tube.19. The catalytic burner of claim 14 wherein the manifold a hub having aplurality of spokes extending therefrom.
 20. The catalytic burner ofclaim 14 wherein at least a portion of the reformation reactor ispositioned within the porous media.